On Tue, May 05, 2020 at 04:09:16PM +0100, Will Deacon wrote:

On Tue, May 05, 2020 at 03:15:29PM +0100, Daniel Thompson wrote:

quoted

On Mon, May 04, 2020 at 09:48:04PM +0100, Will Deacon wrote:

quoted

On Mon, May 04, 2020 at 06:05:18PM +0100, Daniel Thompson wrote:

quoted

diff --git a/arch/arm64/include/asm/cacheflush.h b/arch/arm64/include/asm/cacheflush.h
index e6cca3d4acf7..ce50c1f1f1ea 100644
--- a/arch/arm64/include/asm/cacheflush.h
+++ b/arch/arm64/include/asm/cacheflush.h

@@ -79,7 +79,7 @@ static inline void flush_icache_range(unsigned long start, unsigned long end)
 	 * IPI all online CPUs so that they undergo a context synchronization
 	 * event and are forced to refetch the new instructions.
 	 */
-#ifdef CONFIG_KGDB
+
 	/*
 	 * KGDB performs cache maintenance with interrupts disabled, so we
 	 * will deadlock trying to IPI the secondary CPUs. In theory, we can

@@ -89,9 +89,9 @@ static inline void flush_icache_range(unsigned long start, unsigned long end)
 	 * the patching operation, so we don't need extra IPIs here anyway.
 	 * In which case, add a KGDB-specific bodge and return early.
 	 */
-	if (kgdb_connected && irqs_disabled())
+	if (in_dbg_master())

Does this imply that irqs are disabled?

Yes.

Except for bugs...

quoted

Assuming CONFIG_KGDB is enabled then in_dbg_master() expands to:

    (raw_smp_processor_id() == atomic_read(&kgdb_active))

Aha, so this can drop the raw_ prefix and call smp_processor_id() instead?

We need to allow in_dbg_master() to be called from preemptible contexts
(because its job it to disclose information about our executions
context) but given irqs are always disabled when we in_dbg_master()
then I think we can make this and rely on short-circuit eval to
avoid PREEMPT_DEBUG errors:

    (irqs_disabled() && (smp_processor_id() == atomic_read(&kgdb_active)))

I can queue the arm64 patch regardless.

I don't want to hide anything... when I looked closer I realized
that the above change also eliminates a small window where the original
macro can spuriously evaluate to true.

Specifically if we migrate to a new core after reading the processor
id and the previous core takes a breakpoint then we would evaluate
true if we read kgdb_active before we get the IPI to bring us to halt.

Sorry for overlooking this in my reply yesterday! I'll have a patch out
for this shortly.


Daniel.

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